1. Field of the Invention
The present invention relates to a steering apparatus for use with a pitman arm steering mechanism, the steering apparatus having a variable steering angle ratio, i.e., the variable ratio of an angle through which steerable road wheels are turned to an angle through which a steering wheel is turned.
2. Description of the Prior Art
Pitman arm steering mechanisms comprise a sector shaft rotatable with a steering wheel and a pitman arm angularly movable by the sector shaft, the pitman arm being operatively coupled to steerable road wheels of a motor vehicle. When the steering wheel is turned, the pitman arm is angularly moved by the sector shaft to turn the road wheels for thereby steering the motor vehicle.
Some pitman arm steering mechanisms include a torque sensor for detecting a steering torque applied to the steering wheel, and a hydraulic actuator combined with a steering gear mechanism such as a ball screw mechanism between the sector shaft and the steering wheel. The pitman arm steering mechanisms with the torque sensor and the hydraulic actuator operate as an integral power steering system. The hydraulic pressure supplied to the hydraulic actuator is controlled based on the steering torque detected by the torque sensor, and the hydraulic actuator generates an assistive force to assist in the steering action.
It is known in the art that a steering angle ratio, i.e., the ratio of an angle through which the road wheels are turned to an angle through which the steering wheel is turned, affects the ease with which the motor vehicle can be maneuvered, the stability of the motor vehicle when it runs straight, and the feeling that the driver has in steering the motor vehicle. Japanese patent application No. 3-25553 (corresponding to UK patent application GB 2252282A) discloses a steering apparatus having a mechanism for varying the steering angle ratio. The disclosed mechanism for varying the steering angle ratio may possibly be incorporated in a pitman arm steering apparatus.
Another known steering apparatus with a variable steering angle ratio is disclosed in Japanese laid-open utility model application No. 54-38664. The disclosed steering apparatus includes a steering shaft rotatable by a steering wheel, a nut coupled to the input shaft through a ball screw mechanism which converts rotary motion to linear motion, a lever engaging the nut through a cam mechanism for converting linear motion of the slider to rotary motion, and a cross shaft fixed to the lever. The cross shaft is operatively coupled to road wheels through a pitman arm, for example. The point at which the cam mechanism engages the lever is displaceable to vary an effective radius of the lever for thereby varying a steering angle ratio. The cam mechanism includes a pin mounted on the nut and slidably engaging in a cam groove defined radially in the lever. The pin slides in pressing engagement with a surface of the cam groove. Since a large load is imposed on the pin, the pin tends to wear soon. When the pin is worn, it does not move smoothly in the cam groove, resulting in a reduction in control accuracy, and it also suffers reduced durability.